Many existing video tape recorders, such as the Sony BVU-200A, for example, are constructed to operate at discrete speeds under the control, for example, of a video tape editor such as the Sony BVE-500A. However, in video tape editing processes, and in general, it is often desirable to operate such discrete speed video tape recorders at variable speeds under the control of a manually operable joystick or other manually operable variable speed control, such as is incorporated into video tape editors presently being constructed and sold by the present Assignee, Convergence Corporation. The control system of the present invention provides a simple and economical means by which discrete speeds video tape recorders, such as the Sony BVU-200A, may be controlled to achieve variable speeds, this being accomplished without the need for any modification of the video tape recorder itself.
The Sony BVU-200A video tape recorder, for example, is capable of being controlled to a variety of discrete speeds, such as "still" or "pause" (zero), 1/20 manual play speed (1/20X), 1/5 normal play speed (1/5X), normal play speed (X), and twice normal play speed (2X). The BVU-200A video tape recorder responds to encoded commands from the BVE-500A video tape editor, or from other control sources, to be established at any one of the foregoing discrete speeds. A control is also included in the BVU-200A video tape recorder for establishing either forward or reverse direction of the video tape at any one of the selected discrete speeds.
As mentioned above, the control system of the present invention selects two of the discrete speeds available in the BVU-200A video tape recorder, for example, such as (1/20X), and (2X), and switches between these speeds in a manner which permits a continuously variable speed control of the BVU-200A video tape recorder from, for example, 1/20 normal play speed (1/20X) to twice normal play speed (2X). The continuously variable speed control may be achieved by the joystick of the Convergence video tape editor, as also mentioned above.
In the practice of the invention, the analog control voltage from the joystick is applied to an amplifier which transforms the joystick analog voltage which varies from V- to V+ through zero, to a voltage which only varies between zero and V+. The latter voltage is applied to a constant frequency duty cycle generator which transforms the voltage from the absolute amplifier into a fixed frequency digital waveform whose duty cycle varies as the analog input is changed by adjustment of the joystick. The digital waveform is used to switch between the two selected discrete speeds of the video tape recorder. The result of the controlled switching between the two speeds is a continuously variable tape speed in the video tape recorder as the duty cycle of the digital waveforms changes.
A sample-and-hold circuit may be included to add a "cruise" feature which is incorporated into the Convergence ECS-100 editor, and which causes the tape in the video tape recorder to move at any selected speed for any desired interval without requiring that the joystick control be held in a fixed position for the duration of the interval. Instead, the joystick control may be released, and thereby returned to its zero position, and the tape will continue to move forward or reverse at the preset speed until the cruise function is released, as described in detail in copending application Ser. No. 23,759 filed Mar. 26, 1979, which is assigned to the present Assignee.